1. Field of the Invention
The present invention relates to tracking in hard disk drives, and, in particular, to compensation for tracking error.
2. Description of the Related Art
Bit-patterned media is a technology that magnetic hard drive manufacturers are developing to move to the next level of density for hard disk storage. Bit-patterned media employs diamond-like carbon (DLC)-coated magnetic-head surfaces. A key requirement for enabling the advantages of this technology is the precise control of the write-time and write-head position so that increasingly smaller areas of magnetic media might be written precisely as the write-head passes over the top of these areas.
To maintain circumferential timing synchronization while writing data to the hard disk, timing synchronization marks are included with tracks of the hard disk and are used to maintain phase lock with the media, similar to present methods employed with servo wedges to stay aligned with a track. Since the read-head used to process these timing marks has a variable track offset from the write-head that depends on radial position due to the skew angle of the head arm, a small frequency error might be introduced when the read head and write head are not lined up on the same track. The amount of this error depends not only on the track offset between the read-head and the write-head, but also depends on the center of rotation offset from the center of fabrication of the hard disk.
Usually, this small frequency error resulting from track offset between the read head and the write head is negligible, and has been ignored for hard disk densities of the prior art. When the track offset between read head and write head is ignored in a system where the center of rotation is not exactly the same as the original center of track formation or fabrication, a small frequency and phase error, or center slip, is introduced at the outer diameter or inner diameter of the disk when the read head is used to maintain timing lock while the write head is used to write new data. However, this phase error might be a barrier to achieving the higher density required of bit-patterned media.